In the desulfurization of flue gases and in the production of phosphoric acid, a waste gypsum is produced which is a significant problem with respect to disposal. The term "waste gypsum" is here used to describe the solid product which is a residue of the desulfurization of flue gases in power plants and elsewhere or which is produced in phosphoric acid processes and which consists of calcium sulfate and mixtures of calcium sulfate and calcium sulfite or mixtures of these with other residues or calcium sulfite itself as a residue from such processes.
From German Patent DE-C 32 22 721, a process for transforming the waste gypsum to SO.sub.2 -containing gas and cement clinker is known in which the gypsum is first converted to an anhydrite, e.g. by drying and calcination,and the anhydrite is mixed with additives useful in the production of cement clinker. The decomposition is carried out in a rotary furnace or kiln which generates a product gas from the cleavage or splitting of the calcium sulfate or calcium sulfite which is high in SO.sub.2 and carbon dioxide, the solid residue CaO reacting with the additives to form the cement clinker.
In this case, the decomposition of the anhydrite is carried out under reducing conditions and the reducing agent is coke.
For this process to be effective, it is important to insure a high degree of decomposition of the anhydrite and this can also insure that the permissible sulfate concentration in the clinker does not exceed a maximum permissible level. If the sulfate content of the clinker exceeds such maximum, melting occurs during the clinker formation and the clinker cannot then be readily milled to form the cement clinker. As a consequence, the high quality of the clinker depends on a high degree of dissociations of the anhydrite.
This could not always be ensured with earlier processes.